title: A systematic review and meta-analysis of the global seasonality of norovirus.
abstract: BACKGROUND
  Noroviruses are the most common cause of acute gastroenteritis across all ages worldwide. These pathogens are generally understood to exhibit a wintertime seasonality, though a systematic assessment of seasonal patterns has not been conducted in the era of modern diagnostics.
  
  
  METHODS
  We conducted a systematic review of the Pubmed Medline database for articles published between 1997 and 2011 to identify and extract data from articles reporting on monthly counts of norovirus. We conducted a descriptive analysis to document seasonal patterns of norovirus disease, and we also constructed multivariate linear models to identify factors associated with the strength of norovirus seasonality.
  
  
  RESULTS
  The searched identified 293 unique articles, yielding 38 case and 29 outbreak data series. Within these data series, 52.7% of cases and 41.2% of outbreaks occurred in winter months, and 78.9% of cases and 71.0% of outbreaks occurred in cool months. Both case and outbreak studies showed an earlier peak in season-year 2002-03, but not in season-year 2006-07, years when new genogroup II type 4 variants emerged. For outbreaks, norovirus season strength was positively associated with average rainfall in the wettest month, and inversely associated with crude birth rate in both bivariate and multivariate analyses. For cases, none of the covariates examined was associated with season strength. When case and outbreaks were combined, average rainfall in the wettest month was positively associated with season strength.
  
  
  CONCLUSIONS
  Norovirus is a wintertime phenomenon, at least in the temperate northern hemisphere where most data are available. Our results point to possible associations of season strength with rain in the wettest month and crude birth rate.
authors.affiliation: Epidemiology Branch, Division of Viral Diseases, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia, United States of America ; Department of Environmental Health, Emory University, Atlanta, Georgia, United States of America.
authors.forename: Sharia M
authors.initials: SM
authors.lastname: Ahmed
authors.forename: Benjamin A
authors.initials: BA
authors.lastname: Lopman
authors.forename: Karen
authors.initials: K
authors.lastname: Levy
mesh_terms: D000818:Animals; D038622:Internationality; D029322:Norovirus / Q000502:physiology*; D012621:Seasons